There is a desire at present to make use of optical fibers in ever more numerous and varied applications. Thus, it is desired to be able to use a single optical fiber to convey signals of different kinds, such as telephone signals, television signals, or indeed computer data.
In conventional manner, the signals of different kinds conveyed in the same fiber are given different respective wavelengths specific to each signal, so as to enable the signals to be distinguished from one another.
Thus, it is necessary to be able to use a single optical fiber to convey multiple signals simultaneously, including signals propagating in opposite directions, and it is thus necessary to be able to perform various different operations at an end of the optical fiber.
A first operation is controlling both-way communications over an optical fiber, such as a telephone call, for example.
A second operation is to separate various signals reaching said end, as a function of their wavelengths.
In addition to both-way communications, provision may be made at the end of the fiber to receive a plurality of signals of different wavelengths, such as, for example, a television signal and a signal carrying computer data.
A third operation is multiplexing various signals. This operation is necessary when it is desired to inject various signals of different kinds into the fiber.
For the first operation, proposals are made in "1.31-1.55 .mu.m phase array demultiplexer on InP" by R. Mestric, H. Bissessur, B. Martin, and A. Pinquier, published in IEEE Photonic Technology Letters, Vol. 8, No. 5, May 1996, to use a spectrograph or "grating" comprising an array of waveguides (also known as a "phasar") that has an "inlet" channel on a first coupler and two "outlet" channels on a second coupler, and to connect the two "outlet" channels respectively to a laser and to a photodetector.
The inlet channel is connected to an optical fiber, and the role of the phasar is to inject into the fiber signals which the phasar receives on a first "outlet" channel, and to inject into its other "outlet" channel the signals it receives from the optical fiber.
That system is restricted to two wavelengths that are relatively far apart, i.e. equal to 1.33 .mu.m and 1.55 .mu.m, and it is not capable of separating wavelengths that are separated by differences of less than 0.04 .mu.m, as would otherwise be desirable for both-way communications such as telephone calls.
Thus, that system cannot provide both-way communications at 1.28 .mu.m and at 1.32 .mu.m.
Concerning the first operation, proposals have also been made in "Horizontal directional coupler filter suitable for integration in a 1.3+/1.3- .mu.m duplexer" by S. Frangois, M. Filoche, F. Huet, S. Fouchet, G. Herve-Gruyer, A. Ougazzaden, J. Brandon, N. Bouadma, M. Carre, and A. Carenco, published in Electronics Letters, Vol. 31, No. 23, Nov. 9, 1995, to make a meandering directional coupler enabling both-way communications to be managed on two wavelengths, respectively of 1.28 .mu.m and 1.32 .mu.m, in a telecommunications window situated around a wavelength of 1.3 .mu.m. In order to separate the two wavelengths with good crosstalk between the arms of the coupler, use is made in that case of a Hamming function to vary a coupling coefficient along a propagation direction.
For systems that are small enough in size to be acceptable, directional couplers present the major drawback of being capable of achieving good crosstalk levels only over particularly narrow wavelength zones. Those systems do not make it possible to work over wave length zones that are large.
Furthermore, those systems present the drawback of being particularly difficult to implement.
Concerning the second operation, it is known that a phasar can be used having an inlet channel connected to the optical fiber, and outlet channels connected to receivers where the phasar is adapted to separate signals of different wavelengths reaching the phasar via the fiber, and to deliver them to corresponding receivers connected to the various outlet channels.
Proposals have also been made in "Demonstration and application of a monolithic two-PONs-in-one device" by Yuan P. Li, L. G. Cohen, C. H. Henry, E. J. Laskowski, and M. A. Cappuzzo, of Lucent Technologies, Bell Laboratories, at the 22nd European Conference on Optical Communication, for a system comprising Mach-Zender type elements making it possible, starting from a multiwavelength signal comprising eight channels at 1.5 .mu.m and one channel at 1.3 .mu.m, to subdivide the 1.3 .mu.m channel and to demultiplex the signals at 1.5 .mu.m. That system has eight outlet fibers, each of which delivers a respective portion of the 1.3 .mu.m signal together with one of the eight channels at 1.5 .mu.m.
Mach-Zender type systems suffer from the same drawbacks as both-way couplers. They enable good cross-talk to be obtained only over particularly narrow wave-length zones, and they are particularly difficult to make.
In addition, those various systems do not enable effective separating or multiplexing of signals having wavelengths distributed so that two first wavelengths are particularly close to each other, and a third of said wavelengths is particulary remote from the two first wavelengths. Thus, those systems do not enable effective separating or multiplexing of three signals having wavelenghts respectively of 1.28 .mu.m, 1.32 .mu.m, and 1.54 .mu.m.
In addition, the phasars that have been proposed in the past do not allow waveguides to be disposed close together in their arrays, particulary in the central portion.
Furthermore, current phasars only allow a small number of waveguides to be adopted in their arrays, although many applications require phasars having a high number of waveguides in their arrays.
Known phasars also only allow a waveguide to be adopted in which the difference in length between consecutive waveguides is large.